Optical sensors and specifically photo-mixing devices suffer from a great impact of dark current from e/h pair-generation, for example, at an oxide interface inherent to the photogate structure. For example shot noise limits the signal-to-noise ratio (SNR). Moreover, sensor saturation occurs when integration nodes are used for read-out. Since both effects increase at higher temperatures, the upper operating temperature is limited by exponentially increasing dark current. Separating the photo-generated signal from the dark current may require complex signal analysis.
A further issue is the redirection of the photo-generated charge carriers. To redirect the photo-generated charge carriers, the photogates are located adjacent to a depleted silicon region. Therefore, expensive processes and tools are used to control the doping concentration in the bulk (EPI) such as a semiconductor substrate. Furthermore, the need of a depleted silicon region to shift the photo-generated charge carrier limits the depth of the absorption region in which good carrier redirection, in terms of a demodulation contrast, can be achieved with high demodulation frequency. Moreover, photogates located in the optical path may result in an optical absorption or an optical reflection of the photogate, especially at short wavelengths.
Thus, the current design of optical sensors employing photogate structures, such as photo-mixing devices, suffer from essential limitations of the quality of the sensor signal caused by the design of the sensors.